U.S. Pat. No. 4,929,409 discloses an apparatus for manufacturing a piping element. Innermost in the piping element, there is a flow pipe and around it an insulator. Outside the insulator, there is a corrugated outer sheath. The solution of the U.S. patent can be applied to the manufacture of piping elements that have two or more flow pipes. The flow pipes are then placed in a longitudinal insulation profile having grooves or slots for the pipes. The produced piping element is very good in ring stiffness and is, therefore, especially well suited for use in underground installations, such as district heating networks. The piping element is also flexible and can be coiled for storage and transport. During installation, the piping element is unwound from the coil. The flow pipes then endeavour to move in the longitudinal direction. Coupling the pipes is, therefore, quite difficult and some extra working reserve need to be left at the end of the piping element for safety's sake.
In district heating use, piping elements are also known, in which polyurethane foam is sprayed outside the flow pipes, and an outer sheath is arranged outside the polyurethane foam. Because of the polyurethane foam, this type of piping element is rigid and unbending and its transport, installation and handling is quite arduous and difficult. In addition, when coupling the piping element, the polyurethane foam outside the flow pipe needs to be stripped away, which is quite arduous and difficult. When the polyurethane foam is stripped, a possible oxygen diffusion protection layer arranged outside the pipe is quite easily detached, which weakens the working characteristics of the piping element considerably.